1. Field of the Invention
The present invention relates to a tracking control apparatus used in a magnetic disk apparatus or the like.
2. Description of the Related Art
In conventional magnetic disk apparatuses, recording and reproducing widths of heads were reduced as the track density was raised. Tracking control was performed to cope therewith. That is to say, servo areas each having a plurality of servo patterns as one set are disposed on a track to generate a position error signal, and a command is supplied to an actuator so that this position error signal may become small, a recording and reproducing head being thus positioned on a desired data track.
A conventional tracking control apparatus will now be described by referring to drawings. FIG. 9 is a configuration diagram of a tracking control apparatus using sector servo performed by servo areas respectively disposed on sectors of a data track. The same components as those of FIG. 1 which shows an embodiment are denoted by like numerals.
A signal on a disk reproduced by a recording and reproducing head 1 is amplified by an amplifier 2 and then inputted to timing signal generation means 3 and reproduced output level measurement means 7. FIG. 2 exemplifies data tracks and servo areas written on a disk. As shown in FIG. 2, a reproduced signal obtained when the recording and reproducing head 1 passes becomes a train of pulses having time intervals predetermined by a servo mark, and the magnitude of its amplitude is proportionate to the overlapping width of the recording and reproducing head and the servo mark. FIG. 6 shows the configuration of the timing signal generation means 3 whereto the reproduced signal supplied from the amplifier 2 is inputted. The timing signal generation means 3 comprises measurement timing signal generation means 20, servo mark detection means 21, and servo area detection means 22. The measurement timing signal generation means 20 generates a servo output measurement timing signal 4 functioning as timing in measuring the magnitude of the reproduced output on the servo mark. In response to this, the reproduced output level measurement means 7 is activated to generate a servo level signal 8. Further, the servo mark detection means 21 detects a train of pulses predetermined as the servo mark out of the reproduced signal, generates a servo mark discrimination signal 5 indicating the classification of that servo mark, and outputs it to servo signal storage means 9. The servo signal storage means 9 stores the servo level signal 8 and the servo mark discrimination signal 5 in association with the servo mark. The servo area detection means 22 detects one cluster of servo areas and outputs a position error signal generation timing signal 6 to position error signal generation means 10. On the basis of data of the servo signal storage means 9, a position error signal 11 is calculated. Control means 12 applies predetermined computation to this position error signal 11 and issues a command of movement value to a head actuator 13 for moving the recording and reproducing head 1.
The method for calculating the position signal 11 will now be described. FIG. 3 shows dependence of the magnitude of the servo level signal 8 outputted by the reproduced output level measurement means 7 upon the magnitude of the off-track quantity x between the track center and the recording and reproducing head when the recording and reproducing head passes through the servo mark. Data tracks are written at track intervals T.sub.p, and servo marks A and B are written at a displacement of T.sub.p /2. The width H.sub.2 of the servo marks is equivalent to the core width of the recording and reproducing head at the time of writing. Assuming now that the core width at the time of reading is H.sub.1, the servo level signals obtained when the servo marks A and B are passed are signals which do not change in magnitude for a width of .vertline.H.sub.1 -H.sub.2 .vertline./2 as respectively represented by V.sub.a (x) and V.sub.b (x). In this case, H.sub.1 is not equal to H.sub.2. Even if H.sub.1 is equal to H.sub.2 , however, the following description holds true.
Assuming that the recording and reproducing head is off track by x, the servo level signals are respectively represented as ##EQU1##
At this time, the position error signal is calculated by ##EQU2## From the relation ##EQU3## we get ##EQU4##
Since T.sub.p, H.sub.1 and H.sub.2 have predetermined values, the position error signal x.sub.1 can be derived. The control means 12 issues a command value to the head actuator 13 so that x.sub.1 may be made small. It is thus possible to accurately position the recording and reproducing head 1 on the data track.
As heretofore described, it is possible to derive a position error signal and perform positioning in the above described conventional tracking control apparatus as well.
However, the above described conventional tracking control apparatus has a problem that it is necessary to predetermine the core width of the recording and reproducing head at the time of servo pattern writing and reading in order to calculate the position error signal as described above. That is to say, when a servo pattern is to be written by using the prior art, lowering in tracking characteristics caused by dispersion of position error signals of individual magnetic disk apparatuses is suppressed by positioning the recording and reproducing head accurately and making the dispersion of the core width at the time of working as small as possible. As the track density becomes higher in recent years, however, the core width becomes narrower and the influence of the working precision on the position error signal becomes large. Further, precise working is difficult technically as well, resulting in a higher cost. Further, various contrivances are executed in order to perform accurate writing when a servo pattern is to be written onto a disk. Thus, special provisions are necessary or it takes a time to perform writing, resulting in problems. Further, in case a magnetic disk apparatus having an exchangeable disk such as a floppy disk apparatus has a self-formatting function of writing a servo pattern with each drive, dispersion in track pitch and core width becomes further larger and the precision of the position error signal is extremely aggravated.